Bandgap voltage reference generators are widely used in a variety of applications from analog and mixed signal circuits such as high precision comparators and A/D converters, to digital circuits such as dynamic random access memory (DRAMs) circuits and non-volatile memory circuits. Bandgap voltage references produce a stable voltage reference having a low sensitivity to temperature by generating voltages and/or currents having positive and negative temperature coefficients, and summing these positive and negative coefficients in a manner that creates a temperature stable voltage reference. Traditionally, bandgap voltage references are fabricated using bipolar devices. For example, by summing a signal related to the base-emitter voltage VBE of a bipolar transistor having a voltage inversely proportional to temperature with a signal that is proportional to a difference between the base-emitter voltages ΔVBE of two bipolar transistors that have a voltage proportional to temperature, a temperature stable voltage can be produced as about 1.2 volts, which is about the bandgap of silicon.
The constant trend of microelectronics toward smaller size devices having smaller chip areas has resulted in a corresponding reduction of the maximum allowable supply voltage. For example, some CMOS technologies support a maximum supply voltage of about 1.2 V, which is very close bandgap voltage of silicon. As a result, bandgap references have been adapted to operate under lower voltage conditions to support these lower supply voltages. As supply voltages have continued to be reduced below 1V and begin to approach the nominal base-emitter voltages of silicon bipolar transistors, maintaining headroom within the bandgap voltage has become more challenging.